Send the fork to Mendon for a service....
If you really want a different fork check out your manual in which it should state what the maximum axle to crown length for your frame is....I'm guessing that most 100mm forks will be ok though.....

What perplexes me is that I asked my LBS what maintenance was needed on this shock, and they said nothing aside from air. I never knew I had to worry about oil or anything.

Trying to figure out how I could have damaged it, I am not very aggressive. Hmm.

Well, sorry to say it, but your shop didn't do you any favors.

Cannondale recommends a once a year service. I find you can go longer if you ride less than a few times a week. The tuning of rebound etc, is done via your request at service time. Otherwise, the normal set up should be adequate. If it isn't it may just be that the thing needs servicing....

If you owned it since new and ride as you say, it's unlikely it has internal damage.